In the fields in particular of food products and of oral care, the intention is to administer to an individual a compound such as an active principle or an aromatic product. Various methods of administration have been developed, depending on the applications: powders, granules, capsules, microcapsules, liposomes, microspheres, etc. These various means generally all exhibit characteristics relating to the protection and/or to the release of the compound administered, suitable for the finished product. They may in particular be physical (mechanical, temperature, etc.), chemical, (pH, water, ionic strength, etc.) or enzyme resistance, or properties of insolubility or of gradual solubility (compounds which must be swallowed, gastro-resistant compounds, etc.).
Interest will be focused more particularly here on capsules or microcapsules. Schematically, a capsule or microcapsule comprises a core containing the compound(s) to be vehicled [the content], and one or more envelopes, often based on gelatin. The difference between a capsule and a microcapsule is the size, which is more or less larger. In the present patent application, the term “capsule” will be used to denote capsules and microcapsules.
The capsules known in the prior art can exhibit certain problems of stability and leaktightness of the envelope, with in particular migration of core constituents to the outside or migration of envelope constituents to the inside. Other problems encountered relate to the incorporation of one or more of the components into the core or the envelope of the capsule, or further the method of production.
The capsules encountered are generally swallowed so as to release an active agent in the stomach for example, or, on the contrary, must be kept in the mouth for a certain amount of time in order to allow the active agent or the flavour to be released.
Mention is made herein of U.S. Pat. No. 5,300,305, which describes a microcapsule for oral hygiene comprising a core surrounded by an envelope which contains the active ingredients difficult to solubilize in the core. U.S. Pat. No. 5,370,864 describes a capsule for oral hygiene comprising a particular component in the core. U.S. Pat. No. 5,286,496 describes a microcapsule for oral hygiene in which the core and the envelope comprise active agents.
Certain patents refer to a capsule with several layers. Document EP 1 020 177 describes a soft capsule comprising a first capsule in a second capsule, the second capsule dissolving in the stomach. U.S. Pat. No. 6,200,603 describes a capsule comprising a core and an envelope provided with an additional coating, which stabilizes the capsule and prevents migration of certain compounds between the layers.
The problem of the rate at which a capsule breaks up has been studied. Specifically, for some applications, it is preferable for the capsule to break up rapidly, in order for the compound(s) transported to be rapidly available.
Mention is in particular made herein of U.S. Pat. No. 5,620,707, which describes small hard capsules suitable for flavoring a beverage, presented as dissolving rapidly in the beverage. Capsules produced according to this patent are mentioned hereinafter, by way of a comparative example.
However, in addition to the rate of breaking up, the solubilization of the capsule and more particularly of the envelope of the capsule should be taken into account. The question raised is therefore what becomes of the envelope once it has broken up and released its contents. In fact, in many cases, the capsule envelope is slowly or incompletely insolubilized due to the internal or external crosslinking of the gelatin which it comprises. This effect prevents or slows down the rapid release of the content of the capsule and may, in addition, cause an unpleasant feeling in the mouth due to the nonsolubilized envelope residues, which will be referred to here as the “skin effect”.
Now, it is important, for certain applications, to be free of this skin effect, which causes an unpleasant feeling in the mouth.
U.S. Pat. No. 6,238,690 mentions this problem. In the description of the prior art of this patent, it is specified that capsules are conventionally formed using, for the envelope, film-forming materials of the gelatin type which have the drawback, besides the need to use a complex method of production, of dissolving slowly while leaving an unpleasant residue in the mouth. In this patent, the solution provided is to use, as envelope material, not a film-forming material, but a carbohydrate in the glass state.
However, the problem remains for capsules in which the envelope comprises a film-forming material.